This invention relates to snowmobiles, and more particularly to the rear suspension of a snowmobile suspension systems.
Tracked vehicles such as snowmobiles have rear suspensions generally consisting of front and rear suspension arms pivotally mounted on shafts, which are rotatably connected to a slide frame. The slide frame comprises a pair of laterally spaced apart slide rails or longitudinal skids interconnected transversely on opposing lateral sides of the snowmobile. The slide rails are in sliding contact with an endless track which contacts the snow surface and drives the snowmobile. The front and rear suspension arms pivotally interconnect the chassis of the snowmobile to the slide frame.
As is well known, snowmobiles travel over a wide range of terrain. It is not uncommon for a snowmobile to encounter unexpected and abrupt changes in that terrain. The suspension system must offer sufficient stiffness to absorb and dissipate the forces and energy associated with these abrupt terrain changes. Additionally, aggressive riding of the snowmobile, such as becoming airborne, may require an even greater stiffness and improved shock performance. To provide adequate stiffness for extreme terrain conditions and aggressive riding, while at the same time providing a smooth ride when travelling over relatively smooth surfaces, it is particularly desirable to provide a suspension system that affords a fairly large degree of suspension xe2x80x9ctravel.xe2x80x9d Suspension xe2x80x9ctravelxe2x80x9d refers to the vertical distance through which the suspension collapses upon application of a force from terrain conditions or aggressive riding. Due to the space limitations, it has been difficult to provide a suspension system that provides a wide degree of suspension travel with adequate stiffness throughout the entire range of travel.
When shock absorbers are included in the suspension in simple fashion without extra linkages, as is common practice in the prior art, the shock absorbers produce what is known in the industry as a xe2x80x9cfalling rate,xe2x80x9d referring to the fact that the speed of the shock absorbers decreases, or decelerates, during compression of the suspension. This occurs because, in the conventional system, as the slide frame moves toward the snowmobile chassis, i.e., the shock collapses, the displacement of the shock relative to the slide frame decreases. The falling rate is quantified by the ratio of the displacement of the shock to the displacement of the slide frame, which is referred to as the xe2x80x9cmotion-ratio.xe2x80x9d A smaller displacement of the shock in the same period of time (i.e., a low motion-ratio) translates into a slower shock xe2x80x9cabsorber speed.xe2x80x9d Because the damping force in a shock absorber is directly proportional to the velocity of the shock absorber piston, as the shock absorber speed decreases, the damping force, or reaction force of the shock, also decreases. This falling rate geometry has been looked upon somewhat disfavorably in the prior art because, in a typical prior art suspension having a maximum of 7 inches of travel or less (between the suspension arms and the slide rails), there is a tendency for the suspension to xe2x80x9cbottom outxe2x80x9d more easily. In other words, the suspension arms come into contact with the slide rails.
One design approach used to alleviate the tendency of a shock absorber to bottom out is to provide a suspension system with a shock absorber having a progressive dampening effect, such as that disclosed in U.S. Pat. No. 5,881,834, which is incorporated in its entirety herein by reference. A progressive dampening effect is achieved by progressively decreasing the flow area through which the by-pass fluid is routed during the stroke of the piston within the shock absorber. In the ""834 patent, a helical, tapered passage is formed along the side of the interior wall of the shock absorber cylinder so that when the shock absorber is compressed or extended, the area of the passage decreases, resulting in increased by-pass flow restriction. This, in turn, forces more fluid to travel through the normal valving mechanism, thus providing a progressive dampening effect that helps prevent bottoming-out of the suspension.
Another design approach to prevent bottoming-out is to provide a shock absorber system that contains a progressive rate (i.e., a xe2x80x9crising ratexe2x80x9d design). In a progressive rate system, the speed of the shock absorber increases as the suspension system collapses and, therefore, the force required for successive increments of compression increases. Progressive rate suspension systems are usually found in racing-type snowmobiles with complicated linkage arrangements that manipulate the shock absorber travel into progressively faster shock speed during suspension travel. Such systems are disclosed in U.S. Pat. Nos. 5,727,643 and 4,462,480. However, with conventional snowmobile suspensions, it has been difficult to incorporate an arrangement that provides a progressive spring rate without the use of complex linkages.
Therefore, a need has developed for a snowmobile suspension system with a reduced falling rate (i.e., a substantially constant motion-ratio), without the use of complex linkages, that provides adequate stiffness for both extreme and smooth terrain conditions.
Accordingly, the present invention provides a snowmobile suspension that includes a slide frame for engagement with an endless track, a frame element that connects to a chassis on the snowmobile, a suspension arm, a lower arm assembly, and a shock absorber. The lower arm assembly has a lower end and an upper end, with the upper end pivotally mounted relative to the slide frame. The suspension arm has an upper end pivotally mounted relative to the frame element and a lower end pivotally mounted to the lower end of the lower arm assembly. The shock absorber has an upper end pivotally mounted relative to the frame element and a lower end pivotally mounted to the lower end of the lower arm assembly. The upper end of the suspension arm is positioned forward of and below the upper end of the shock absorber and pivots independently of the upper end of the shock absorber. The upper end of rear shock absorber is positioned adjacent the frame element and the lower end of the shock absorber is positioned adjacent the slide frame. The mounting positions defined by (i) the upper end of the suspension arm, (ii) the upper end of the shock absorber, (iii) the lower end of the suspension arm, and (iv) the lower end of the shock absorber cooperate to provide a substantially constant motion-ratio as the slide frame collapses toward the frame element.
Other features and advantages of the present invention will be realized in accordance with the following detailed description, appended drawings, and claims.